Aircraft and other vehicles are commonly provided with cavity-backed slot antennas which in general involve a slot through a ground plane. These cavity-backed slot antennas are by their very nature narrow banded and it is only with difficulty that one can increase the bandwidth of the slot antenna so that it may be used over a wide frequency range for detection of multiple transmitters. Wide bandwidth slots support, for instance, direction finding involving angle of arrivals (AOA) determinations, and radar-warning systems. Unlike horn and spiral antennas, slots can be spaced within a half wavelength to allow unambiguous phase determination, beamforming and sidelobe control.
Moreover, it is desirable to provide an S band or an L band conformal slot antenna for high power communications. In general slots may be scaled dimensionally to support systems of various mission needs. Combining ultra-wide bandwidth with the scalability and phase control of slot arrays allows them to be used with the most demanding radar and communications signal receiving electronic-warfare receivers and transmitters.
The slot is cut or etched into a metallic ground plane, which may be shaped to conform to the smoothly curving surface of an aircraft or other platform, thus being described as conformal.
In the past one way to obtain greater bandwidth was to attempt to increase the width of the slot. However, the result is a wide open wave guide cavity. The problem with such a wide slot is an intolerable radar cross section that would cause a stealthy platform to be susceptible to illumination by enemy radars.
Another application that was attempted was to create an array of four slots in a square arrangement so that the resulting antenna would not only have a broader bandwidth, but also would behave like a monopole extending out of the surface of the ground plane. If the four slots were fed together in phase, while one would achieve a monopole behavior, the bandwidth would nonetheless be limited by the bandwidth associated with the slots. Other feed arrangements for the square array allow diversity of polarization or direction finding. However, these also would be of limited application if the slot bandwidth could not be extended.
The challenge was to come up with a way to make a fat slot but with the fat slot mostly covered up so as not to present a large structural radar cross section. Moreover, there needed to be a way to fit the slots in a square array without the wide fat slots overlapping.
In short, a topology needed to be developed that would provide a 2:1 or 3:1 bandwidth without significantly increasing the platform's radar cross section and to do so with conformal antenna apertures usable on the skin of aircraft or other vehicles.
In summary, there is a conflict between close spacing and minimum length and width for slot antennas in a quad array. Close half-wavelength spacing or less is required element-to-element at highest frequencies (i.e., short wavelengths), but the length of elements must be half-wavelength at low frequencies (long wavelengths) for efficient performance. Also, each element must be wide enough to achieve bandwidth. Additionally, the radar cross section of the conformal aperture must be minimized. A need therefore exists for wide instantaneous bandwidth (3:1) conformal slot apertures capable of handling high power and of being arrayed in a quad configuration for 360 degree azimuthal coverage.